1. Field of the Invention
The present invention relates to an optical pickup as used in a data storage device such as an optical or magneto-optical disk device to record and reproduce data to and from a disk-type data storage medium.
2. Description of the Prior Art
FIG. 4 shows an example of the construction of a conventional optical pickup. This optical pickup operates as follows. A semiconductor laser chip 1 emits a laser beam in accordance with a current supplied from a laser driving circuit 2, which supplies the current to the semiconductor laser chip 1 in accordance with a control signal (voltage signal) produced by an automatic power control circuit 11. As the laser beam emitted from the semiconductor laser chip 1 is shone onto a disk 100 used as a data storage medium, it is made into a light spot S on the disk 100 through an optical system, which also serves to direct the light reflected from the disk 100 to a light-reception photodiode 4.
The light-reception photodiode 4 converts the received light into a current signal, and this current signal is further converted into a voltage signal by a current-to-voltage converter 5 for light reception. On the other hand, part of the laser beam emitted from the semiconductor laser chip 1 (more specifically, that part of the laser beam which is emitted from the side of the semiconductor laser chip 1 opposite to the optical system 3) is converted into another current signal by a monitoring photodiode 6.
The automatic power control circuit (hereafter referred simply as the "APC circuit") 11 feeds the control signal to the laser driving circuit 2 in accordance with the current signal fed from the monitoring photodiode 6 in such a way that the semiconductor laser chip 1 emits the laser beam at a constant power. That is, the automatic power control circuit 11 provides feedback control of the semiconductor laser chip 1.
In the optical pickup, the APC circuit 11 is indispensable for the following reason. In the optical pickup, it is essential that its "signal output level", that is, the level of the voltage signal obtained from the optical pickup (i.e. from its current-to-voltage converter 5 for light reception), be kept at a specific level. However, it is impossible to keep constant the power of the laser beam emitted from the semiconductor laser chip 1 simply by controlling it in a fixed and stable manner all the time, because the power of the laser beam not only depends on the characteristics of the individual semiconductor laser chip 1, but also varies greatly with ambient temperature. Thus, it is impossible to keep constant the signal output level of the optical pickup without using the APC circuit 11.
On the other hand, in general, the "conversion efficiency" of a photodiode, that is, the efficiency with which the photodiode converts a light signal into a current signal, depends heavily on the characteristics of the individual photodiode. This often causes the light-reception photodiode 4 and the monitoring photodiode 6 to have different conversion efficiency and, in addition, the degree to which they differ (i.e. the difference in the conversion efficiency) varies from one optical pickup to another. As a result, whereas it is possible at least to keep the signal output level constant as long as the APC circuit 11 has a fixed input/output characteristic, it is not always possible to keep the signal output level at a specific level since the level at which it is fixed differs from one optical pickup to another.
To overcome this problem, the APC circuit 11 of the above conventional optical pickup is so designed that its input/output characteristic is adjustable. Specifically, as shown in FIG. 4, the APC circuit 11 typically includes a variable resistor 111, a constant voltage source 72, and an operational amplifier 73 that has its non-inverting input terminal (+) connected to the constant voltage source 72 and its inverting input terminal (-) connected to the variable resistor 111. The operational amplifier 73 receives, at its non-inverting input terminal (+), a predetermined voltage and, at its inverting input terminal (-), a voltage signal obtained by converting the current signal from the monitoring photodiode 6 by means of the variable resistor 111, and feeds its output to the laser driving circuit 2. Thus, the input/output characteristic of the APC circuit 11 can be varied by varying the resistance of the variable resistor 111.
As described above, a conventional optical pickup needs to be provided with an APC circuit whose input/output characteristic is adjustable. This means that a conventional optical pickup requires an extra component, i.e. a variable-resistance circuit, to allow adjustment of the input/output characteristic of its APC circuit, and that the production of a conventional optical pickup requires an extra process for adjusting the input/output characteristic of its APC circuit to set its signal output level at a specific level. Quite inconveniently, this has been posing a limit on cost reduction of optical pickups.